In most cases older parish registers like these, at least for Church of England churches, will be held by the local county archives/record office. In Cornwall that would be the Cornwall Record Office.
There's a catalogue on the Cornwall Record Office site that you can search to see what they have, but that is just an index of what registers they hold, not of the actual contents of those registers so you would need to start with some idea of what parish you might need to look at.
There are register transcriptions on Find My Past however, and a quick look found the marriage you are interested in. Robert Chapman and Elizabeth Shapling are listed as marrying on 5 Aug 1828 in the parish of Werrington.
Going back to the Cornwall Record Office catalogue we can see that they hold the Werrington parish registers under reference P251. You might notice that they list Werrington as Devon in the catalogue - it seems it is right on the border of Devon and Cornwall and was in Devon until 1966 when most of it was transferred to Cornwall.
The only Robert Chapman that I can see a baptismal record for within five years of 1811 is in the parish of St Stephen by Launceston in 1807 and Werrington is a small village a couple of miles north of Lanceston so that seems like a good candidate for further research.
As @Chadwi has pointed out, the Cornish registers are also on Family Search, complete with images.The marriage record is present but unfortunately it doesn't give the names of any of the parents.
There don't seem to be any baptism registers listed for Werrington before 1813 so if Robert was baptised there then it may need further work to determine if the registers survive. The registers for St Stephens by Launceston also seem to be largely missing from Family Search unfortunately.